Capacitive touch sensor systems are used to implement a variety of useful functions, including touch sensor devices (e.g., touch pad, touch dial, touch wheel, etc.), for determining the presence of an object, accelerometers, other functions. Capacitive touch sensor devices have been incorporated into a range of electronic devices, including mobile communication devices, such as cellular telephones, computers, portable entertainment devices, appliances, and touch screens, to name but a few. A capacitive touch sensor system includes one or more capacitive touch sensor devices, each of which is configured to indicate a capacitance change when, for example, the sensor device is touched (e.g., by a stylus or a human finger). Such changes in capacitance may then be processed by the system in order to detect when the sensor device has been touched.
In a conventional capacitive touch sensor system, a capacitance of the sensor device is sampled, and a baseline value is subtracted from the sampled value to derive a delta value. The delta value is then compared to a threshold value. If the delta value is greater than the threshold value, then a touch is detected, whereas if the delta value is less than the threshold value, then no touch is detected.
In order to accommodate for changes in ambient operating conditions and sensitivity changes, it may be important to dynamically update the baseline value. However, applications of capacitive touch sensor systems are often subjected to large amounts of electromagnetic noise effects, which can cause significant disturbances to the sampled capacitance signal. Specifically, such disturbances can cause noise-related fluctuations in the sampled capacitance signal. Such noise fluctuations can result in the delta value for the sampled signal dipping below the threshold value during a touch event, giving a false “not touch detected” reading. Such false “not touch detected” readings result in inappropriate tracking of the baseline value to the sampled signal value during a touch event. As a consequence, the baseline value is inappropriately increased, increasing the vulnerability of the touch sensing components to noise fluctuations, and reducing the ability of a capacitive touch sensor device to accurately detect touches.
In order to minimize the effects of electromagnetic noise, filtering may be applied to the sampled data prior to deriving the delta value. However, such filtering can reduce the sensitivity of the capacitive touch sensor device components, making setting of the threshold value difficult for reliable touch recognition.